This invention relates to a resistor chip structured by forming not only an electrically resistant layer on the surface of an electrically insulating substrate in the form of a chip but also terminal electrodes on both end parts of this substrate.
Prior art resistor chips of this type were usually produced firstly by forming an electrical resistor film on the upper surface of an insulating substrate, upper-surface electrodes of a silver material on both end parts and a glass cover coating so as to entirely cover the resistant layer. Next, side-surface silver electrodes are formed on the side surfaces of the substrate so as to be electrically connected to these upper-surface electrodes, and a plated metallic layer comprising a nickel layer serving as a lower layer and a tin or solder layer serving as an upper layer is formed on each of the upper-surface and side-surface electrodes.
With a prior art resistor chip thus structured, because the surfaces of its silver upper electrodes are covered in part by a glass cover coating and in part by a plated metallic layer, a force comes to be concentrated along the boundaries between the cover coating and the plated metallic layer on the upper electrodes due to the difference between their coefficients of thermal expansion. Thus, cracks tend to be easily formed along these boundary portions, not only causing the resistance of the resistor to vary but also separating the upper electrodes with the resistor film.
In view of such problems, Japanese Patent Publication Tokkai 4-237102 proposed extending both end parts of the resistor film beyond the cover coating on the upper side of the upper-surface electrodes and forming a plated metallic layer also on the surface of the extended portions. According to the disclosure therein, however, the width of the resistor film is made smaller than that of the upper-surface electrodes. Thus, the outer portions of the upper-surface electrodes are not covered by the extended portion of the resistor film, and boundaries between the glass cover coating and the plated metallic layers on the upper-surface electrodes were in part on the surface of these parts.
As a result, probability remained to exist for the occurrence of cracks in the portions of the upper-surface electrodes outside the extended parts of the resistor film due to the aforementioned difference in the coefficient of thermal expansion. Such cracks are likely to extend even to the portions covered by the resistor film. Moreover, gaps are likely to result between the plated metallic layers and the cover coating on the outer portions of the upper-surface electrodes beyond the extended parts of the resistor film due the difference between their coefficients of thermal expansion. When the resistor chip is used in an atmosphere of a corrosive gas, the upper-surface electrodes at such gaps become corroded by such a gas. If the corrosion extends to the area covered by the extended parts of the resistor film, this certainly will affect the resistance value of the resistor chip.
It is therefore an object of this invention in view of the problems described above to provide a resistor chip of an improved structure.
A resistor chip embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising an electrically insulating substrate in the form of a chip, a pair of upper-surface electrodes on the upper surface of the substrate and mutually separated from each other, an resistor film having end portions which are each over a corresponding one of these upper-surface electrodes, a cover coating made of a glass material which is over a portion of the resistor film, and a pair of plated metallic layers each over an end surface of the substrate but also wherein edge sections of the resistor film over the upper-surface electrodes are not covered by the cover coating and are each directly covered by one of the plated metallic layers. The upper-surface electrodes and the end portions of the resistor film may be rectangular, for example, with the width of the upper-surface electrodes made smaller than the width of the end portions of the resistor film.
With a resistor chip thus structured, with the portions of the upper-surface electrodes abutting the cover coating completely covered by the widened end portions of the resistor film such that the forces caused by the difference between the coefficients of thermal expansion of the upper-surface electrodes and the glass cover coating can be dispersed over the end portions of the resistor film. Thus, the upper-electrodes can be dependably prevented from becoming corroded even if the resistor chip is used in an atmosphere of a corrosive gas and a gap is generated between the plated metallic layers and the cover coating because the upper-electrodes will not be exposed through such a gap. As a result, the present invention can provide highly reliable resistor chip with no changes in the resistance value.